linux-kernel - Re: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG

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Message-ID: <uc3g3fgwirwczxjbh5qxgz3pzqmlmiymdeh7m2dzznx2fap4vc@6hvvrgpbyg5q>
Date: Mon, 19 May 2025 08:10:50 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	linux-kernel@...r.kernel.org, Nicolas Pitre <npitre@...libre.com>, 
	Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG_ON() for
 divide by zero

On Sun, May 18, 2025 at 02:38:46PM +0100, David Laight wrote:
> Do an explicit BUG_ON(!divisor) instead of hoping the 'undefined
> behaviour' the compiler generated for a compile-time 1/0 is in any
> way useful.
> 
> It may be better to define the function to return ~(u64)0 for
> divide by zero.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
> 
> A new change for v2 of the patchset.
> Whereas gcc inserts (IIRC) 'ud2' clang is likely to let the code
> continue and generate 'random' results for any 'undefined bahaviour'.
> 
>  lib/math/div64.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index a5c966a36836..c426fa0660bc 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -186,6 +186,9 @@ EXPORT_SYMBOL(iter_div_u64_rem);
>  #ifndef mul_u64_u64_div_u64
>  u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  {
> +	/* Trigger exception if divisor is zero */
> +	BUG_ON(!d);
> +

I'm unsure if I should like the BUG_ON better than return 1/0. My gut
feeling is that mul_u64_u64_div_u64() should behave in the same way as
e.g. div64_u64 (which is just `return dividend / divisor;` for 64bit
archs and thus triggers the same exception as `return 1/0;`.

If BUG_ON should be it, I'd prefer

	BUG_ON(unlikely(d == 0));

which keeps the unlikely() that is already in the check removed below
and is more explicit that checking for !d.

>  	if (ilog2(a) + ilog2(b) <= 62)
>  		return div64_u64(a * b, d);
>  
> @@ -212,13 +215,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  
>  #endif
>  
> -	/* make sure d is not zero, trigger exception otherwise */
> -#pragma GCC diagnostic push
> -#pragma GCC diagnostic ignored "-Wdiv-by-zero"
> -	if (unlikely(d == 0))
> -		return 1/0;
> -#pragma GCC diagnostic pop
> -
>  	int shift = __builtin_ctzll(d);
>  
>  	/* try reducing the fraction in case the dividend becomes <= 64 bits */

Best regards
Uwe

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